polymers Article
Fluorescent Amorphous Distyrylnaphthalene-Based Polymers: Synthesis, Characterization and Thin-Film Nanomolar Sensing of Nitroaromatics in Water Raúl O. Garay * , Ana B. Schvval, Marcela F. Almassio, Pablo G. Del Rosso, Maria J. Romagnoli and Rosana S. Montani INQUISUR, Departamento de Química, Universidad Nacional del Sur (UNS)-CONICET, Av. Alem 1253, Bahía Blanca B8000CPB, Argentina;
[email protected] (A.B.S.);
[email protected] (M.F.A.);
[email protected] (P.G.D.R.);
[email protected] (M.J.R.);
[email protected] (R.S.M.) * Correspondence:
[email protected]; Tel.: +54-291-459-5101 Received: 20 November 2018; Accepted: 6 December 2018; Published: 10 December 2018
Abstract: Two new fluorescent segmented conjugated polymers with either 1,4- or 2,6-distyrylnaphthalene chromophores and their model compounds were synthesized and the chemosensing abilities of the polymeric thin films to detect nitroaromatics (NACs) in aqueous media were evaluated. The structural, thermal and optical properties of the polymers were correlated with those displayed by their corresponding model compounds. Changes in the connectivity of naphthylene units caused minor differences in optical properties, morphology and quenching efficiencies. Molecular modeling highlighted the extremely bent character of polymer microstructures that explains their high solubility and amorphous character. Polymeric films are amorphous, strongly fluorescent and showed remarkable quenching efficiencies in the nanomolar range with picric acid (PA) and trinitrotoluene (TNT). Quenching experiments using either different nitroaromatic quenchers, excitation wavelengths, excitation beam path-lengths, or time of exposure of the film to the quenching solution evidenced the dominant role of inner filter effects (IFE) in the polymer response to NACs in the micromolar range. The sensing response towards PA, a quencher that strongly absorbs at the excitation wavelength, has an IFE contribution even at the nanomolar range, while the response towards the non-absorbing TNT depends only on the quenching occurring after diffusion of the analyte into the film. Keywords: Segmented conjugated polymer; amorphous; thin films; fluorescent sensor; nitroaromatics; nanomolar detection
1. Introduction Fluorescent organic materials have a broad range of sensing applications [1,2] and exist in a variety of architectures [3], ranging from small molecules [4] to chromophoric assemblies, such as monolayers [5], macromolecules [6] or dendrimers [7]. Among them, thin films of fluorescent polymers have raised much interest for chemo-/bio-sensing [8] due to their ease of fabrication, mechanical and chemical stability, tunable shape and portability, as well as their success in detecting vapors of explosive compounds as a result of an amplified response [9]. In addition to the intrinsic electro-optical properties of the polymer, permeation of analytes across polymer films, that depends on both morphology and polymer-analyte interactions, is essential in solid-state sensing configurations where luminescent films respond to trace amounts of analytes which are either in the vapor phase or in solution [8,9]. We have previously evaluated the capabilities of thin films of segmented conjugated polymers (SCPs) to detect environmental pollutants presenting a hazard to ecological and human health that are
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We have previously evaluated the capabilities of thin films of segmented conjugated polymers (SCPs) to detect environmental pollutants presenting a hazard to ecological and human health that present in water bodies of industrial and city such assuch nitroaromatic compounds (NACs)(NACs) [10,11]. are present in water bodies of industrial andareas, city areas, as nitroaromatic compounds In order to obtain amorphous morphologies that could favor diffusion of the analyte from the [10,11]. In order to obtain amorphous morphologies that could favor diffusion of the analyte from aqueous solution into into the polymeric film, film, we knitted semi-rigid fluorophores with isopropylene units, the aqueous solution the polymeric we knitted semi-rigid fluorophores with isopropylene which confer a highly twisted nature to the main chain. We found that thin films of these disordered units, which confer a highly twisted nature to the main chain. We found that thin films of these assemblies fluorophores proved to proved be veryto sensitive inNACs aqueous media [12,13]. disordered of assemblies of fluorophores be very towards sensitive NACs towards in aqueous media Indeed, we and many others have found that different fluorescent materials [14–24], despite their [12,13]. Indeed, we and many others have found that different fluorescent materials [14–24], despite huge disparity in chemical structure, are very responsive in aqueous media towards picric acid their huge disparity in chemical structure, are very responsive in aqueous media towards picric (PA) acid and or ano, small, to trinitrotoluene (TNT) and related Nevertheless, (PA)show and no, show or responses a small, responses to trinitrotoluene (TNT) nitrotoluenes. and related nitrotoluenes. itNevertheless, has been difficult to establish generaltostructure-property for the quenching responses of it has been difficult establish generalrelationships structure-property relationships for the NACs that could be used to improve the performance and selectivity of the materials by rational design. quenching responses of NACs that could be used to improve the performance and selectivity of the Some of the arise from the of occurrence of different competing quenching materials bydifficulties rational design. Some the difficulties ariseorfrom the occurrence of mechanisms different or other than photoinduced electron transfer (ET) [3], such as resonance energy transfer (RET) competing quenching mechanisms other than photoinduced electron transfer (ET) [3], [3,16] such or as inner filter effects (IFE) [25–27]. resonance energy transfer (RET) [3,16] or inner filter effects (IFE) [25–27]. In In this this report, report, we we first first describe describe the the synthesis synthesis and and characterization characterization of of the the fluorescent fluorescent segmented segmented conjugated polymers P14 and P26, with either 1,4or 2,6-distyrylnaphthalene chromophores, conjugated polymers P14 and P26, with either 1,4- or 2,6-distyrylnaphthalene chromophores, and and compounds M14 and M26 (Scheme 1). In additionremarkable to showing remarkable theirtheir modelmodel compounds M14 and M26 (Scheme 1). In addition to showing photophysical photophysical the distyrylnaphthalene moieties onto can the be main tethered main properties, the properties, distyrylnaphthalene moieties can be tethered chainonto withthe different chain with different regio-connectivity, which could modify optical properties, morphology and regio-connectivity, which could modify optical properties, morphology and quenching efficiencies. quenching The new and their model compounds were characterized by gel The new efficiencies. polymers and theirpolymers model compounds were characterized by gel permeation permeation chromatography (GPC), differential scanning calorimetry (DSC), NMR spectroscopy, chromatography (GPC), differential scanning calorimetry (DSC), NMR spectroscopy, FT-IR FT-IR spectroscopy, UV-Vis spectroscopy, fluorescence spectroscopy.The Thestructural, structural, thermal thermal and spectroscopy, UV-Vis spectroscopy, andand fluorescence spectroscopy. and optical properties of the polymers were correlated with those displayed by their corresponding model optical properties of the polymers were correlated with those displayed by their corresponding compounds. Finally,Finally, we evaluated the chemosensing abilitiesabilities of the polymeric thin films detect model compounds. we evaluated the chemosensing of the polymeric thintofilms to various hydrophilic and hydrophobic nitroaromatics in aqueous media. media. detect various hydrophilic and hydrophobic nitroaromatics in aqueous H3CO 1a
t-Bu t-Bu OCH3
t-Bu
CHO OCH3
PO(OEt)2
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(EtO)2OP
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CHO OCH3 CH3 CH3
CHO
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Scheme 1. 1. Synthetic routes routes to to polymers polymers and and their their model model compounds. compounds. Scheme
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2. Materials and Methods 2.1. General Methods and Instrumentation The general methods and instrumentation used in the structural, thermal and optical characterization are reported in Supplementary Materials. Several hydrophobic (i.e., 2,4,6-trinitrotoluene, TNT; 2,4-dinitrotoluene, DNT; 4-nitrotoluene, NT) and hydrophilic (i.e., picric acid, PA; 2,4-dinitrophenol, DNP; 4-nitrophenol, NP) nitroaromatics were used. Fluorescence control experiments were previously done by checking during 24 h the occurrence of polymer leakage from the film towards the aqueous media; P14 and P26 films supported on quartz or glass plates showed no bleeding. 2.2. Materials and Synthesis Reagents (Sigma-Aldrich, Darmstadt, Germany) were used without further purification. DMF was dried over molecular sieves and stored under argon atmosphere. THF was distilled from Na/benzophenone. 1,4-Bis(bromomethyl)naphthalene and 2,6-Bis(bromomethyl)naphthalene were obtained by radical bromination of 1,4-dimethylnaphthalene and 2,6-dimethylnaphthalene, respectively. The reaction of the dibromides with triethylphosphite afforded the bisphosphonates 1a and 1b [28]. Aldehyde 2 and dialdehyde 3 were prepared according to the literature [13]. The details of the synthesis and characterization of the synthetic precursors, monomer, model compounds M14 and M26 and the polymers P14 and P26 are presented in Supplementary Materials. 2.3. Molecular Modeling The ORCA 4.0 program package (Max Planck Institute for Coal Research, Mülheim, Germany) [29] and the graphical interface Gabedit 2.4.8 (Université Claude Bernard Lyon1, Villeurbanne, France) [30] were used. Initial structures were obtained with the AM1 method and then structure optimization was carried out using the PBEh-3c method [31]. This DFT method is based in a modified PBE functional, uses Ahlrichs-DZ basis sets and the “3c” stands for the use of a gCP correction, a dispersion correction (D3) and minor modifications of the basis sets. It has been designed to compute structures and interaction energies of large systems and could reach results close to that of the MP2/TZ level with a small fraction of the computational cost. 3. Results and Discussion 3.1. Synthesis Synthetic routes for the new polymers P14 and P26 with bis(styrylnaphthylene) units tethered by isopropylene units and its model compounds M14 and M26 are shown in Scheme 1. The Horner-Emmons-Wittig coupling to obtain carbon-carbon double bonds with predominant E configuration was used. First, bisphosphonates 1a and 1b were prepared from the 1,4- and 2,6-bis(bromomethylene)naphthalenes, respectively. Then, monoaldehyde 2 and dialdehyde 3 were used as coupling partners of 1a and 1b to obtain M14 and M26 and the polymers P14 and P26. No precipitate or gel formation was detected when THF:DMF (1:1) was used as the polymerization reaction media. But early precipitation and lower degrees of polymerization were observed when the coupling reaction was run in pure THF. Benzaldehyde and diethyl benzylphosphonate were used as end-capping compounds to terminate polymerizations. Despite the lack of solubilizing long alkyl side chains, P14 and P26 were soluble in organic solvents such as CHCl3 , CH2 Cl2 , DMF, toluene or THF so the crude polymers were precipitated from their chloroform solutions with methanol to give yellow solids. GPC analysis indicated that good degrees of polymerization were reached (P14, Mn = 18.7 kDa, Mn /Mw = 2.4, DPn = 43; P26, Mn = 5.5 kDa, Mn /Mw = 2.4, DPn = 13). NMR and FTIR spectroscopies confirmed the all-trans stereochemistry and chemical structures of the coupling products (Figure 1).
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1 H spectra of P14 and P26 showed ethenyl CH signals above 7.00 ppm with coupling constants JThe trans = 16.1 Hz in P14 and Jtrans = 13.5 Hz in P26, while no signals were observed in the region of 6.5 Jtrans which = 16.1 would Hz in P14 and Jtrans 13.5cis-counterparts Hz in P26, while no signals were observed in the region of ppm correspond to=their (Figure 1). Likewise, when compared to their 6.5 ppm which wouldcounterparts, correspond tothe their cis-counterparts compared to respective polymeric expanded aromatic (Figure regions1). of Likewise, the modelwhen compounds M14 their respective polymeric counterparts, the expanded aromatic regions of the model compounds and M26 showed quite similar values of chemical shifts and Jtrans coupling constants and no M14 and M26signals showed similar of chemical the shifts and Jtransregularity coupling of constants and no cis-ethylenic at quite ca. 6.5 ppm,values thus confirming structural the polymeric cis-ethylenic signals1,atexpanded ca. 6.5 ppm, thus confirming structural regularityinofall thecoupling polymeric structures structures (Figure aromatic regions). the Besides, the presence products of (Figure 1, expanded aromatic regions). Besides, the presence in all coupling products of an intense IR an intense IR band at ≈ 960 cm−1 (out-of-plane C–H deformation band) of trans-substituted vinylene −1 (out-of-plane C–H deformation band) of trans-substituted vinylene groups and band atand ≈960 groups thecm absence of the characteristic absorbance of cis-substituted alkene at ≈ 730 cm−1 were the absence of the characteristic of cis-substituted alkene at ≈730 cm−1 were additional additional indications of the highabsorbance content of trans-configurations. indications of the high content of trans-configurations.
(a)
(b)
1 13 Figure Figure 1. 1. 1H H NMR NMR (CDCl (CDCl33)) (top) (top) and and 13CCNMR NMR (CDCl (CDCl33with with5% 5%DMF) DMF) (bottom) (bottom) spectra spectra of of polymers polymers 11H NMR spectra of the expanded aromatic region of both model compounds (a) P14 and (b) P26. The (a) P14 and (b) P26. The H NMR spectra of the expanded aromatic region of both model compounds 13 and and polymers polymers are are also also shown. shown. ** 13CCsignal signalfrom fromDMF-d DMF-d7. .
7
3.2. Morphology. Morphology. Thermal Thermal and and Optical Optical Properties Properties The results results of of the the DSC DSC analysis analysis are are shown shown in inFigure Figure2a 2aand andTable Table S1 S1 (Supplementary (SupplementaryMaterials). Materials). ◦ distyrylnaphthylene M14 M14 exhibited exhibited aa strong glass transition transition (29 (29 °C) C) and and a broad melting The new distyrylnaphthylene ◦ C) in the first heating cycle, but cooling of the melt led to the formation of a glassy endotherm (74 (74 °C) endotherm in the first heating cycle, but cooling of the melt led to the formation of a glassy ◦ C) which persisted in the subsequent heating cycles. M26 showed a strong glass transition state (19 °C) which persisted in the M26 showed a strong ◦ C) ◦ C) (140 °C) and a broad melting endotherm (220 °C) in the first heating cycle. A broad crystallization ◦ C) was present only in the first cooling which was followed by the glass transition. exotherm (167 (167 °C) exotherm was present only in the first cooling which was followed by the glass transition. Then, M26 exhibited only strong glass transitions upon repetitive heating and cooling cycles in DSC plots. Occasionally, weak cold crystallization followed by weak melting transitions was observed in the heating cycles. Both compounds bearing bulky tert-butyl groups have strong tendencies to form disordered gatherings that render kinetically stable amorphous glasses, though their differences in
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plots. Occasionally, weak cold crystallization followed by weak melting transitions was observed in Polymers 2018, 10, x FOR PEER REVIEW 5 of 13 the heating cycles. Both compounds bearing bulky tert-butyl groups have strong tendencies to form disordered gatherings render kinetically stable amorphous glasses, their in molecular structure arethat manifested in that M14 forms amorphous glassesthough after the firstdifferences heating cycle molecular structure are manifested in that M14 forms amorphous glasses after the first heating cycle while M26 achieves the amorphous state after the second heating cycle. On the whole, the longer while M26 achieves the amorphous after the second heating cycle. On the whole, the shorter longer and and more planar M26 (Figure 2b)state displayed higher transition temperatures than the and more M26 (Figure 2b)traces displayed higher P14 transition temperatures the shorter bulgy bulgyplanar M14 (Figure 2c). DSC of polymers and P26 showed onlythan noticeable glassand transitions M14 (Figure traces of polymers and P26values showed only noticeable glassin transitions at 155 at 155 and 2c). 170 DSC °C respectively, these P14 transition reflect the difference connectivity of ◦ and 170 C respectively, these transition values reflect the difference in connectivity of naphthylene naphthylene groups in the semi-rigid moiety of the polymers again. Moreover, the films appeared groups in theblack semi-rigid of the polymers Moreover, in thethe films appeared range completely black completely when moiety observed by POM withagain. cross polarizers temperature between 50 ◦ C thus when observed by POM with cross polarizers in the temperature range between 50 and 300 and 300 °C thus indicating the occurrence of optically isotropic phases (Figure S1, Supplementary indicating theThin occurrence of optically range isotropic phases (Figure Supplementary Materials). Thin films Materials). films (thickness = 30 to 200 nm)S1,cast from chloroform solutions were (thickness range = 30 to 200 nm) cast from chloroform solutions were transparent, homogeneous and transparent, homogeneous and appropriate for optical measurements. Likewise, thicker films appropriate for optical measurements. Likewise, thicker films continued to be transparent without a continued to be transparent without a sign of birefringence even after several weeks at room sign of birefringence eventhe after several weeksand at room temperature. the highP14 solubility temperature. Evidently, high solubility amorphous natureEvidently, of the polymers and P26and are amorphous nature of the polymers P14 and P26 are rooted in the twisted chain conformations that rooted in the twisted chain conformations that the polymer microstructure must adopt by virtue of the polymer microstructure must adopt by virtue of the angle imposed to the gem-chromophores by the angle imposed to the gem-chromophores by the saturated isopropylene spacer. The the saturated isopropylene spacer.ofThe DFT/PBEh-3c ofits P26highly shownbent in DFT/PBEh-3c molecular model a tetramer of P26molecular shown in model Figureof2da tetramer highlights Figure 2d highlights its highly bent character. character.
M26
Endo>
1HC 2HC 1CC
(b)
(c)
M14 1HC 2HC 1CC
P26 2HC 1CC
P14
2HC 1CC
(d) -50
0
50
100
150
200
250
Temperature, oC
(a) Figure2.2. (a) model compounds and polymers. (b) Figure (a) Differential Differential scanning scanningcalorimetry calorimetry(DSC) (DSC)traces tracesofof model compounds and polymers. DFT/PBEh-3c relaxed geometry of M14, (c) (c) M26 andand (d)(d) a tetramer of P26. (b) DFT/PBEh-3c relaxed geometry of M14, M26 a tetramer of P26.
The morphological morphological information information from from DSC DSC and and POM POM was was complemented complemented with with UV-Vis UV-Vis and and The fluorescencespectroscopy spectroscopy data data that provided information of fluorescence informationabout aboutthe theinteraction interactionand andlocal localordering ordering thethe chromophores [32,33]. Absorption and emission spectraspectra of P14 in exhibitedexhibited small red of chromophores [32,33]. Absorption and emission of chloroform P14 in chloroform shiftsred in comparison to those oftoM14 (Figure 3a(Figure and Table S1, Supplementary Materials).Materials). Therefore, small shifts in comparison those of M14 3a and Table S1, Supplementary the interaction of adjacent chromophores along the isolated polymer chainschains is quite small.small. Both Therefore, the interaction of adjacent chromophores along the isolated polymer is quite absorption spectra, whichwhich are usually very informative about the degree order-disorder present in Both absorption spectra, are usually very informative about theofdegree of order-disorder the solid of M14 max,abs 383 nm) =and max,abs = 382 nm) films in overlap the condensed present in state, the solid state,(λof M14= (λ 383P14 nm)(λand P14 (λmax,abs = 382overlap nm) films in the max,abs phase and phase show no differencesdifferences against those recorded solutionin(Figure 3b). Hence,3b). no condensed anddistinctive show no distinctive against thoseinrecorded solution (Figure significant amounts amounts of chromophores in eitherinthe monomeric or the or polymeric assemblies form Hence, no significant of chromophores either the monomeric the polymeric assemblies locallocal ground-state aggregates within films. Moreover, the fluorescence spectra of aof P14 (λmax,em = 472 form ground-state aggregates within films. Moreover, the fluorescence spectra a P14 (λmax,em nm) film exhibits a small blue-shift against that of M26 (λmax,em = 482 nm), which is due to a shift in the relative weight of the vibronic bands, revealing that excited species generated by chromophores locked in random orientations by the twisted nature of the polymer chains exist in an environment less ordered than that generated by the monomeric chromophores [32,33]. The same conclusions
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= 472 nm) film exhibits a small blue-shift against that of M26 (λmax,em = 482 nm), which is due to a shift in the relative weight of the vibronic bands, revealing that excited species generated by chromophores locked in random orientations by the twisted nature of the polymer chains exist in an Polymers 2018, 10, x FOR PEER REVIEW 6 of 13 environment less ordered than that generated by the monomeric chromophores [32,33]. The same conclusions by a similar comparative the optical properties of chloroform were reachedwere by areached similar comparative analysis of theanalysis optical of properties of chloroform solutions and solutions andand films of(Figure M26 and P26Remarkably, (Figure 3c,d).allRemarkably, all absorption transitions, as well films of M26 P26 3c,d). absorption transitions, as well as the emission as the emission transitions, of these distyrylnaphthalene-based model and polymers lie transitions, of these distyrylnaphthalene-based model compounds andcompounds polymers lie roughly in the roughly in the same region spectroscopic in the thin films showing that the different connectivity did same spectroscopic in the region thin films showing that the different connectivity did not bring not bring profound changes to their optical behavior. Calculations at the DFT level also indicate that profound changes to their optical behavior. Calculations at the DFT level also indicate that minimal minimal differences in themolecular frontier molecular orbital (FMO) of energies of the fluorophores areby brought differences in the frontier orbital (FMO) energies the fluorophores are brought either by either the different connectivity of the aromatic moieties in P14 and P26 or the monomeric versus the different connectivity of the aromatic moieties in P14 and P26 or the monomeric versus the the oligomeric structure (Figure oligomeric structure (Figure 5a).5a).
a
M26 P26
c Absorption/Emision (a. u.)
Absorption/Emision (a. u.)
M14 P14
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M26 P26
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Figure 3. Absorption (left) andand emission (right) spectra. (a,c)(a,c) CHCl 3 solutions, (b,d) Thin films. Figure 3. Absorption (left) emission (right) spectra. CHCl 3 solutions, (b,d) Thin films.
3.3. Fluorescence Fluorescence Response Response to to Nitroaromatic Nitroaromatic Compounds 3.3. Compounds Quenching experiments experiments were were performed performed on on polymer polymer thin thin films supported supported on quartz or glass Quenching substrates which were immersed in water, and their fluorescence response which were immersed in water, and their fluorescenceto increasing response concentrations to increasing of NACs was recorded. Data presented in presented Figure 4 were obtained from fluorescence intensities concentrations of NACs was recorded. Data in Figure 4 were obtained from fluorescence recorded after signal stabilization in the conditions commonly used toused gather fluorescence data, intensities recorded after signal stabilization in the conditions commonly to gather fluorescence
data, that is, films were irradiated (λexc) at or near the maximum of the lowest energy absorption band (λmax,abs) and fluorescence intensities were observed (λobs) at or near the wavelength corresponding to the maximum in the emission spectra (λmax,em) of the sensing polymer. The Stern– Volmer (S–V) relationship, (Io/I) − 1 = KSV [NAC], between the ratio of the fluorescence intensity with and without the NAC, I and Io, and the added analyte concentration, [NAC], was used to quantify
and P26 with nitrophenols and nitrotoluenes in the 0–450 µM concentration range. Table S2 (Supplementary Materials) lists the S–V constant (KSV) obtained from the linear part of the curves presented in Figure 4, which corresponded to regions between 5%–30% to 55%–65% of the total quenching, and the values of half of the maximum quench, Q50%, that is, the quencher concentration Polymers 2018, 10, 1366 7 of 13 needed to reach (Io/I) − 1 = 1. Polymers P14 and P26 showed high sensitivity to increased concentrations of NACs. Despite that is, films were irradiated (λexcbut ) at or the maximum of the energy absorption band their microstructural differences, in near line with the similarity of lowest the optical and morphological (λ ) and fluorescence intensities were observed (λ ) at or near the wavelength corresponding properties of their films, both polymers displayed fairly max,abs obs comparable responses. The quenching to the maximum in the emission spectra higher (λmax,emthan ) of the polymer. for Theboth Stern–Volmer response to nitrophenols is distinctively thatsensing to nitrotoluenes polymers, (S–V) with relationship, KSV [NAC], between the ratiowe of the fluorescence intensity with and without PA and DNP(Ishowing the highest sensitivities. Thus, found that the quenching efficiency order o /I) − 1 = the the NAC, I and Io , and the added [NAC], was used to quantify quenching for most studied NACs is PAanalyte >> TNT,concentration, as has been frequently observed in many the other systems efficiencies Thus, Figure shows plotswith of the S–V equation foroccurred P14 andwith P26 [14–24]. We for alsovarious noticedNACs. that non-linear S–V 4relationships upward curvature with nitrophenols nitrotoluenes in the 0–450 µM concentration range. Table S2 (Supplementary nitrophenols at theand higher end of quencher concentrations. While with most nitrotoluenes, a rather Materials) lists of thethe S–V constant in (Kthe from theregion linear part of the curves presented incurvature Figure 4, steep increase efficiency low micromolar is followed by a downward SV ) obtained which corresponded to regions 5%–30% to 55%–65% of the the values of at higher concentrations, this between phenomena is more noticeable in total P26 quenching, (Figure S2,and Supplementary half of the maximum Q50% , that is, the quencher concentration neededi.e., to reach Materials). Moreover,quench, in this region quenching efficiencies were comparable, TNT ≥(IPA. o /I) − 1 = 1. (a)
(b) P14
P26
TNP DNP NP TNT DNT NT
Q50%
1
0
(Io/I) - 1
2
(Io/I) - 1
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TNP DNP NP TNT DNT NT
Q50%
1
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50?
100?
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[NAC], M
[NAC], M
Figure plots with with nitrophenols nitrophenols and and nitrotoluenes nitrotoluenes (a) (a) for for P14 P14 films films (λ (λexc == 384 nm, Figure 4. 4. The The Stern–Volmer Stern–Volmer plots exc 384 nm, obs = 470 nm) and (b) for P26 films (λexc = 371 nm, λobs = 461 nm). The graphs include a visual aid to λ λobs = 470 nm) and (b) for P26 films (λexc = 371 nm, λobs = 461 nm). The graphs include a visual aid to indicate indicate the the half half of of the the maximum maximum quench, quench, Q Q50%. . 50%
Fluorescence (a.u.)
Absorbance
E (eV)
(a) Polymers P14 and P26 showed high sensitivity (b) to increased concentrations of NACs. Despite their microstructural differences, but in line with the similarity ofλ the (nm)optical = 371/384and morphological properties 0 exc of their films, both polymers displayed fairly comparable responses. The quenching response to 1.0 1.0 TNT P26 P14 with PA and DNP nitrophenols is distinctivelyET higher than that to nitrotoluenes for both polymers, -2 PA showing the highest sensitivities. Thus, we found0.8thatDNT the quenching efficiency order for 0.8 the most NP NT in many other systems [14–24]. We also studied NACs is PA >> TNT, as has been frequently observed -4 0.6 0.6 DNP noticed that non-linear S–V relationships with upward curvature occurred with nitrophenols at the higher end of quencher concentrations. While with most nitrotoluenes, a rather steep increase of the -6 0.4 0.4 M26 micromolar P26T efficiency in the low region is followed by a downward curvature at higher concentrations, P14T M14 this phenomena is more noticeable in P26 (Figure 0.2 S2, Supplementary Materials). Moreover, in this 0.2 -8 NP comparable, i.e., TNT ≥ PA. region quenching efficiencies were NT DNP 0.0 0.0 the low The -10 non-linearity of S–V plots and in quenching efficiencies between DNT the variation PA TNT micromolar and high micromolar range imply that concurrent quenching mechanisms are operative. 300 350 400 450 500 λ, nm In electron-rich structures such as P14 and P26, fluorescence quenching by nitroaromatics that follows their optical excitation is generally attributed to electron-transfer quenching (ET) occurring from the Figure 5. (a) Frontier molecular orbitals energies (DFT/PBEh-3c) of M14, M26 and tetramers (P14T excited polymer to the LUMO of the electron deficient nitroaromatic molecule. In our case, the results and P26T) of P14 and P26. Values for the frontier molecular orbitals (FMOs) of the quenchers are also of the DFT calculations (see Figure 5a) indicate that ET from the excited polymer to the ground state of the nitroaromatics is thermodynamically favorable for all quenchers. Another mechanism of non-radiative excited state depletion involves radiationless energy transfer processes (FRET) [3,34]. Thus, fluorescence quenching could result from either ET or FRET or a combination of both processes. The latter process requires spectral superposition of the absorption spectrum of the quenchers and the
nitrophenols at the higher end of quencher concentrations. While with most nitrotoluenes, a rather steep increase of the efficiency in the low micromolar region is followed by a downward curvature at higher concentrations, this phenomena is more noticeable in P26 (Figure S2, Supplementary Materials). Moreover, in this region quenching efficiencies were comparable, i.e., TNT ≥ PA. Polymers 2018, 10, 1366
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(a)
(b) P14
P26
TNP
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(Io/I) - 1
(Io/I) - 1
DNP fluorescence spectra of the fluorophores. Figure 5b presents the absorption spectraDNP of the nitrophenols NP 2 2 NP and the nitrotoluenes obtained at the same quencher concentration and the fluorescence spectra of TNT TNT DNT both P14 and P26. There it can be observed that among the NACs tested here onlyDNT the strongly acidic NT NT DNP and PA present intense absorption bands with λmax,abs ≈ 355 nm, which partially overlap the fluorescence spectra of P14 and P26, indicating that FRET could be contributing to quenching with Q50% Q50% occurs these1two NACs. On the contrary, no such overlap 1 with NP and the other nitrotoluenes tested here. Moreover, inner filter effects (IFE) must also be considered for absorbing quenchers. Although IFE is not strictly a fluorescence quenching process, a non-fluorescent quencher could absorb either the excitation beam (primary IFE) or the emitted beam (secondary IFE) of the fluorophores or both. Such attenuation of the excitation and emission beams results in an additional contribution to the 0 total 0response of the polymers to NACs [34]. The quenching efficiency order for P26 (see Figure 4 0 50? 100? 150? 0 50? the intensity 100? 150? optical and Table S1, Q50% = PA > DNP > NP >> NT > DNT > TNT) follows of their [NAC], M [NAC], M densities (OD) at the excitation wavelength of P26 (λexc = 371 nm) as highlighted by the dashed vertical bar inFigure Figure 5b. Stern–Volmer It also can be appreciated in Figure that the red infilms the λ(λexc used for P14 4. The plots with nitrophenols and5b nitrotoluenes (a) shift for P14 exc = 384 nm, (λexc = 384 nm) reduces the OD of the quencher NP at the λ significantly, and its quenching efficiency λobs = 470 nm) and (b) for P26 films (λexc = 371 nm, λobs = 461 exc nm). The graphs include a visual aid to also drops accordingly. indicate the half of the maximum quench, Q50%.
(a)
(b) λexc (nm) = 371/384 1.0
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Figure 5.5. (a) (a) Frontier Frontier molecular molecular orbitals orbitals energies energies (DFT/PBEh-3c) (DFT/PBEh-3c) of M14, M26 and and tetramers tetramers (P14T (P14T Figure and and P26T) P26T) of of P14 P14 and and P26. P26. Values Values for for the the frontier frontier molecular molecular orbitals orbitals (FMOs) (FMOs) of of the the quenchers quenchers are are also also included. (b) UV-Vis absorption spectra, [NAC] = 56 mM. Fluorescence spectra of P14 and P26 are also included. Vertical dashed lines indicate the excitation wavelengths used to record fluorescence spectra.
Thus, the role of the excitation wavelength in the quenching efficiencies was examined in more detail. Figure 6b shows the S–V plots of the fluorescence response of P26 to PA, which were obtained with four different excitation wavelengths, λexc , as shown in Figure 6a. Assuming that Kasha–Vavilov’s rule holds [34], the S–V plots should be somewhat similar in magnitude and shape if FRET is dominant since this phenomenon occurs from the lowest excited state whose quantum yield is independent of the excitation wavelength. Indeed, the S–V plots and magnitudes of Ksv values are entirely different, and they distinctly depend on the molar extinction coefficient ε of the quencher PA at each excitation wavelength (See Figure 6a). Thus, the smaller response is obtained at λexc = 285 nm and the larger at λexc = 371 nm, thus reflecting the growing contribution of primary IFE. The increase of the optical density of the quencher solution as the excitation wavelength approaches the maximum of the absorption spectra of PA (λmax,abs = 355 nm) results in increased absorption of the excitation beam. No second IFE contribution is expected in this case because we observe the decreasing fluorescence intensities at a wavelength (λobs = 500 nm) where PA absorbance is null. Moreover, S–V plots become increasingly non-linear as the optical density of the solution at the excitation wavelength increases.
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Likewise, we observed that changing the path-length of the absorption and emission beams inside the cuvette, by placing the film in different sensing geometries, also influenced the film response (Figure S3, Supplementary Materials). Therefore, while the shorter path-length (60◦ S) into the quencher solution yields a linear S–V plot, longer path-lengths (60◦ L and 30◦ ) corresponded to higher quenching efficiencies and increasingly non-linear S–V plots, a typical behavior observed when inner filter effects are operative. Therefore, the increase of the optical density either by changing the excitation wavelength or the path-length of the excitation beam results in an upward curvature. In principle, Polymers 2018, 10, x FOR PEER REVIEW 9 of 13 amplified quenching observed in extended conjugated systems [9], or combined quenching that occurs in the fluorophores confined in a membrane [35], could leadInto our positive areenvironments confined in awhere membrane [35], could are lead to positive deviations from linearity. case, deviations from linearity. In our case, amplified quenching can be ruled out due to the segmented amplified quenching can be ruled out due to the segmented nature of the conjugated system of the nature of the system of themobility polymer along P26 that exciton mobility along the polymer P26conjugated that prevents exciton theprevents polymer backbone. Although wepolymer cannot backbone. Although we cannot rigorously exclude that combined quenching occurs after the quencher rigorously exclude that combined quenching occurs after the quencher diffuses into the film, the fact diffuses theobservations film, the factand thatthe thequenching above observations and the efficiency order of that the into above efficiency order of quenching NACs are correlated with the NACs correlated with the opticalthat density intensities suggestsrole thatinIFE plays a dominant role in opticalare density intensities suggests IFE plays a dominant determining the quenching determining the quenching efficiency order of the polymers at the micromolar concentration range. efficiency order of the polymers at the micromolar concentration range. 335
371
8
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7
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Figure6.6. (a) (a)Absorption Absorptionspectrum spectrumof ofthe thepicric picricacid acidin inwater water(red (reddashed dashedline). line).Absorption Absorptionspectrum spectrum Figure exc = = 371 371 nm) nm) as as aa function function of ofincreasing increasing of aaP26 P26film film(solid (solidline) line)and andfluorescence fluorescencespectra spectrachange change(λ(λexc of concentrationsofofpicric picric acid (PA) in water, fluorescence were recorded 1 min after the concentrations acid (PA) in water, fluorescence spectraspectra were recorded 1 min after the quencher exc used to gather data presented in (b) while the up quencher addition. Down arrows indicate the λ addition. Down arrows indicate the λexc used to gather data presented in (b) while the up arrow to observe the fluorescence (b) The Stern–Volmer arrow indicates λem indicates the λem the used toused observe the fluorescence decrease.decrease. (b) The Stern–Volmer plots for aplots P26 for filma em = and 500 nm) and PA obtained λexc = 285, and 371 nm. P26 (λ = 500(λnm) PA obtained with λexc with = 285, 310, 335,310, and335, 371 nm. em film
Next, order to study the sub-and low micromolar region, thinner films of P26 Next, ininorder to study the sub-and low micromolar region, thinner films of P26 (thickness = 35 (thickness 35 ±used 3 nm)towere used to improve efficiencies quenching efficiencies S4, Supplementary ± 3 nm) = were improve quenching (Figure S4, (Figure Supplementary Materials). Materials). the effectofofP26 thickness P26 films was not systematically Although theAlthough effect of thickness films on of efficiency wason notefficiency systematically investigated in the investigated in the study, we observed the expected by decreasing present study, wepresent observed the expected improvement inimprovement performance in byperformance decreasing film thickness, film with both quenchers showing remarkable quenching the nanomolar withthickness, both quenchers showing remarkable quenching efficiencies in theefficiencies nanomolarinregion, as well as region, as well as similarity in the response towards TNT and (Figure Though the non-linear similarity in the response towards TNT and PA (Figure 7). PA Though the7).non-linear nature of the nature of thecomplicates responses complicates the calculation of the of detection by linear regression, responses the calculation of the limit of limit detection (LOD)(LOD) by linear regression, we we observe 10% of the fluorescence quenched 50 nM concentrations of either or TNT, observe thatthat 10% of the fluorescence waswas quenched by by 50 nM concentrations of either PAPA or TNT, so so LOD should below these concentration values (see insets Figure Thus, sensitivities thethe LOD should bebe below these concentration values (see insets inin Figure 7).7). Thus, thethe sensitivities of of P26 among the best reported the literature polymericsolutions solutionsororfilms films[25,36–39]. [25,36–39]. P26 areare among the best reported inin the literature forfor polymeric Besides, fluorescence intensities were measured at 1, 10, 20 and 30 min after the addition of the aliquot of the quencher solution to estimate both the true quenching (ET and/or FRET) and IFE contributions to the total response. Figure 7a shows that the response of the P26 film to TNT can be exclusively assigned to quenching since the last measurement of fluorescence intensity (after 30 min) at a given concentration and the first (after 1 min) at the next concentration have nearly the same value indicating that there is no sudden signal attenuation due to quencher absorption (i.e., no IFE) after the quencher addition. For each quencher concentration, the film response increases in time due to quenching by diffusion of the NAC into the film. On the other hand, steep growth in the film response occurs immediately after each increment of PA concentration (Figure 7b), which is then
cm−1 (Figure S5, Supplementary Materials). Therefore, a 5 µM solution would already have an optical density of 0.07, which is significant considering that for analytical studies it is recommended to correct IFE above optical densities of 0.02 [40]. As a result, the sensing response of P26 towards PA, a quencher that strongly absorbs at the excitation wavelength, has an IFE contribution even at the nanomolar range, while the response towards the non-absorbing TNT depends only Polymers 2018, 10, 1366 10 ofon 13 quenching.
1.0
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Figure 7. The The Stern–Volmer Stern–Volmerplots plotsforfor PN26 films (λexc = 361 = 500 (a) picric Figure 7. PN26 films (λexc = 361 nm,nm, λobsλobs = 500 nm)nm) withwith (a) picric acid acid (PA) (PA) and (b) trinitrotoluene (TNT). lines Dotted guides the eye. join the last and (b) trinitrotoluene (TNT). Dotted are lines guidesare to the eye. to Dotted linesDotted join thelines last measurement measurement 30 concentration min) at a given concentration and at the 1 min) at the innext (after 30 min) at(after a given and the first (after 1 min) thefirst next(after concentration. Insets the concentration. Insets in the of graphs show an expansion of the scale close to the origin. graphs show an expansion the scale close to the origin.
Besides, fluorescence intensities were measured at 1, 10, 20 and 30 min after the addition of 4. Conclusions the aliquot of the quencher solution to estimate both the true quenching (ET and/or FRET) and IFE Two new fluorescent segmented conjugated polymers with distyrylnaphthalene chromophores contributions to the total response. Figure 7a shows that the response of the P26 film to TNT can be and their model compounds were synthesized. No marked differences in optical properties, exclusively assigned to quenching since the last measurement of fluorescence intensity (after 30 min) morphology or quenching efficiencies were produced by changing the connectivity of naphthylene at a given concentration and the first (after 1 min) at the next concentration have nearly the same units. Supported films of both P14 and P26 were amorphous, strongly fluorescent with no value indicating that there is no sudden signal attenuation due to quencher absorption (i.e., no IFE) aggregation of the chromophores in the solid state and highly sensitive towards NACs in water. In after the quencher addition. For each quencher concentration, the film response increases in time particular, PA and TNT showed remarkable quenching efficiencies at the nanomolar level. due to quenching by diffusion of the NAC into the film. On the other hand, steep growth in the Quenching experiments using either different nitroaromatic quenchers, excitation wavelengths, film response occurs immediately after each increment of PA concentration (Figure 7b), which is then excitation beam path-lengths, or time of exposure of the film to the quenching solution point to a followed by slow growth of the response with time due to quenching following diffusion of the NAC dominant role of inner filter effects (IFE) in the quenching efficiency order of the polymer response into the film. These sudden increments of quenching began to be noticeable at concentrations of to PA, TNT, and other NACs in the micromolar concentration range. It was also observed that the 0.3–0.8 µM and already become very important at the low end of the micromolar region. By adding up sensing response of P26 towards PA, a quencher that strongly absorbs at the excitation wavelength, the quenching increments at each concentration, we estimated that quenching contributes to nearly has an IFE contribution in addition to quenching even at the nanomolar range, while the response 50% of the overall response needed to reach Q50% at [PA] = 10 µM while for TNT, which is devoid of towards the non-absorbing TNT depends only on the quenching occurring after diffusion of the IFE, Q is reached at [TNT] = 16 µM (Figure 7). The observed IFE contributions of PA are rooted analyte50% into the film. Finally, we point out here that the adjustment of the optical properties (λ abs and in the high molar attenuation coefficient of PA in water at λexc = 371 nm, which is 13,680 M−1 cm−1 λem) of the polymer by structural design and the judicious selection of experimental conditions (λex (Figure S5, Supplementary Materials). Therefore, a 5 µM solution would already have an optical and λobs) could be used to maximize or minimize IFE as an additional criteria in order to enhance density of 0.07, which is significant considering that for analytical studies it is recommended to correct sensitivity or selectivity towards the NAC of interest. IFE above optical densities of 0.02 [40]. As a result, the sensing response of P26 towards PA, a quencher that strongly absorbs at the an IFE contribution even at the nanomolar Supplementary Materials: Theexcitation following wavelength, are available has online at www.mdpi.com/xxx/s1, Experimental 1. range, while the response towards the Experimental non-absorbing depends onlyS1. onOptical quenching. General methods and instrumentation, 2. TNT Synthesis, Figure micrograph (cross polarizers) of P26 between glass substrates al 196 °C, Figure S2. Expansion of the scale close to the origin of 4. Conclusions Figure 4. Stern–Volmer plots for a PN26 film with nitrotoluenes, Figure S3. Stern–Volmer plots for a PN26 film exc = 371 nm, λem = 500 nm). The path-lengths of the excitation (blue) and emission (red) beams inside with PA (λnew Two fluorescent segmented conjugated polymers with distyrylnaphthalene chromophores
and their model compounds were synthesized. No marked differences in optical properties, morphology or quenching efficiencies were produced by changing the connectivity of naphthylene units. Supported films of both P14 and P26 were amorphous, strongly fluorescent with no aggregation of the chromophores in the solid state and highly sensitive towards NACs in water. In particular, PA and TNT showed remarkable quenching efficiencies at the nanomolar level. Quenching experiments using either different nitroaromatic quenchers, excitation wavelengths, excitation beam path-lengths,
Polymers 2018, 10, 1366
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or time of exposure of the film to the quenching solution point to a dominant role of inner filter effects (IFE) in the quenching efficiency order of the polymer response to PA, TNT, and other NACs in the micromolar concentration range. It was also observed that the sensing response of P26 towards PA, a quencher that strongly absorbs at the excitation wavelength, has an IFE contribution in addition to quenching even at the nanomolar range, while the response towards the non-absorbing TNT depends only on the quenching occurring after diffusion of the analyte into the film. Finally, we point out here that the adjustment of the optical properties (λabs and λem ) of the polymer by structural design and the judicious selection of experimental conditions (λex and λobs ) could be used to maximize or minimize IFE as an additional criteria in order to enhance sensitivity or selectivity towards the NAC of interest. Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/10/12/ 1366/s1, Experimental 1. General methods and instrumentation, Experimental 2. Synthesis, Figure S1. Optical micrograph (cross polarizers) of P26 between glass substrates al 196 ◦ C, Figure S2. Expansion of the scale close to the origin of Figure 4. Stern–Volmer plots for a PN26 film with nitrotoluenes, Figure S3. Stern–Volmer plots for a PN26 film with PA (λexc = 371 nm, λem = 500 nm). The path-lengths of the excitation (blue) and emission (red) beams inside of the cuvette were changed by placing the film in different sensing geometries: 60◦ L, 30◦ and 60◦ S, Figure S4. Fluorescence spectra change (λex = 371 nm) of P26 films as a function of (a) added TNT in water; [TNT] = 0.05–189 µM (top to bottom). (b) added PA in water; [PA] = 0.05–194 µM (top to bottom), Figure S5. Calculation of the molar attenuation coefficient of PA at λ371, Table S1. Thermal and optical properties of model compounds and polymers, Table S2. Data analysis of Stern–Volmer plots and quenching efficiencies, Q50% , of the fluorescence responses of P14 and P26 to NACs in water. Author Contributions: Experiments were performed by A.B.S. (film formation and quenching studies), M.F.A. (synthesis, fluorescence characterization), M.J.R. (quenching studies), P.G.D.R. (structural characterization), R.S.M. (DSC and optical characterization). All authors contributed to the data analysis. The paper was written and the project was conceived by R.O.G. Funding: This research was funded by SGCYT-UNS grant number 24/Q055 and CIC-PBA grant numbers 1707/15 and MFA/16. Acknowledgments: Financial support from SGCyT-UNS and CIC-PBA is acknowledged. A.B.S. and M.J.R. thank CONICET for a fellowship. M.F.A. is a member of the research staff of CIC-PBA. P.G.D.R. and R.O.G. are members of the research staff of CONICET. Conflicts of Interest: The authors declare no conflict of interest.
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